1. Field of the Invention
This invention relates to a filter device such as a digital filter device having a variable cutoff frequency. Also, this invention relates to a method of acquiring filter coefficients to be set in a filter device.
2. Description of the Related Art
It is known to use a digital signal processor (DSP) to form a filtering section of a digital filter device having a variable cutoff frequency. The filtering section of such a digital filter device contains a memory loaded with a signal of a set of filter coefficients. The actual cutoff frequency of the digital filter device is determined by the set of the filter coefficients.
A first known digital filter device includes a DSP-based filtering section having a variable cutoff frequency, and a ROM storing signals of sets of filter coefficients which correspond to different cutoff frequencies respectively. The ROM is accessed in response to an input command signal representative of a desired cutoff frequency so that a signal of a set of filter coefficients corresponding to the desired cutoff frequency will be read out from the ROM. The read-out signal of the filter coefficient set is written into a memory within the filtering section to equalize the actual cutoff frequency of the filtering section to the desired cutoff frequency. When many signals of sets of filter coefficients are required to be stored in the ROM, the capacity of the ROM needs to be great.
A second known digital filter device includes a DSP-based filtering section having a variable cutoff frequency, and a CPU programmed to calculate a set of filter coefficients. Specifically, the CPU calculates a set of filter coefficients from a desired cutoff frequency represented by an input command signal. The CPU writes a signal of the set of the calculated filter coefficients into a memory within the filtering section to equalize the actual cutoff frequency of the filtering section to the desired cutoff frequency. When accurate and fast calculation of filter coefficients is required, a high-grade expensive CPU is needed.
A background-art digital filter device which is not prior art to this invention includes a DSP-based filtering section having a variable cutoff frequency, a ROM storing signals of sets of first filter coefficients which correspond to different cutoff frequencies respectively, and a CPU programmed to calculate a set of second filter coefficients. The ROM is accessed in response to an input command signal representative of a desired cutoff frequency so that a signal of a set of first filter coefficients corresponding to the desired cutoff frequency will be read out from the ROM. The CPU receives the read-out signal of the set of the first filter coefficients. The CPU calculates a set of second filter coefficients from the set of the first filter coefficients. The CPU writes a signal of the set of the calculated second filter coefficients into a memory within the filtering section to equalize the actual cutoff frequency of the filtering section to the desired cutoff frequency. When many signals of sets of first filter coefficients are required to be stored in the ROM, the capacity of the ROM needs to be great. When accurate and fast calculation of second filter coefficients is required, a high-grade expensive CPU is needed.
It is a first object of this invention to provide an improved filter device.
It is a second object of this invention to provide an improved method of acquiring filter coefficients.
A first aspect of this invention provides a filter device comprising first means for receiving data of a desired cutoff frequency; second means for deciding whether the desired cutoff frequency is in a predetermined low frequency band or a predetermined high frequency band, the predetermined low frequency band being lower in frequency than the predetermined high frequency band; third means for storing data of filter coefficients corresponding to different cutoff frequencies in the predetermined low frequency band; fourth means for storing data of precalculated basic coefficients corresponding to different cutoff frequencies in the predetermined high frequency band, the precalculated basic coefficients being equal to values resulting from a part of calculation to provide final filter coefficients; fifth means for calculating final filter coefficients from precalculated basic coefficients through a coefficient expanding process; a filtering section; sixth means for, in cases where the second means decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from the third means, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in the filtering section; and seventh means for, in cases where the second means decides that the desired cutoff frequency is in the predetermined high frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from the fourth means, causing the fifth means to calculate final filter coefficients from the read-out precalculated basic coefficients, and setting data of the calculated final filter coefficients in the filtering section.
A second aspect of this invention is based on the first aspect thereof, and provides a filter device further comprising first interpolating means for implementing interpolation with respect to the filter coefficients represented by the data read out from the third means, and second interpolating means for implementing interpolation with respect to the data read out from the fourth means or the final filter coefficients calculated by the fifth means.
A third aspect of this invention is based on the first aspect thereof, and provides a filter device further comprising first interpolating means for implementing logarithmic interpolation with respect to the filter coefficients represented by the data read out from the third means, and second interpolating means for implementing logarithmic interpolation with respect to the data read out from the fourth means or the final filter coefficients calculated by the fifth means.
A fourth aspect of this invention provides a filter device comprising first means for receiving data of a desired cutoff frequency; second means for deciding which of a predetermined low frequency band, a predetermined intermediate frequency band, and a predetermined high frequency band the desired cutoff frequency exists in, the predetermined low frequency band being lower in frequency than the predetermined intermediate frequency band, the predetermined intermediate frequency band being lower in frequency than the predetermined high frequency band; third means for storing data of filter coefficients corresponding to different cutoff frequencies in the predetermined low frequency band; fourth means for storing data of precalculated basic coefficients corresponding to different cutoff frequencies in the predetermined intermediate frequency band, the precalculated basic coefficients being equal to values resulting from a part of calculation to provide final filter coefficients; fifth means for calculating final filter coefficients from precalculated basic coefficients through a coefficient expanding process; sixth means for calculating filter coefficients from a cutoff frequency in the predetermined high frequency band; a filtering section; seventh means for, in cases where the second means decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from the third means, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in the filtering section; eighth means for, in cases where the second means decides that the desired cutoff frequency is in the predetermined intermediate frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from the fourth means, causing the fifth means to calculate final filter coefficients from the read-out precalculated basic coefficients, and setting data of the calculated final filter coefficients in the filtering section; and ninth means for, in cases where the second means decides that the desired cutoff frequency is in the predetermined high frequency band, causing the sixth means to calculate filter coefficients from the desired cutoff frequency, and setting data of the calculated filter coefficients in the filtering section.
A fifth aspect of this invention is based on the fourth aspect thereof, and provides a filter device further comprising first interpolating means for implementing interpolation with respect to the filter coefficients represented by the data read out from the third means, and second interpolating means for implementing interpolation with respect to the data read out from the fourth means or the final filter coefficients calculated by the fifth means.
A sixth aspect of this invention is based on the fourth aspect thereof, and provides a filter device further comprising first interpolating means for implementing logarithmic interpolation with respect to the filter coefficients represented by the data read out from the third means, and second interpolating means for implementing logarithmic interpolation with respect to the data read out from the fourth means or the final filter coefficients calculated by the fifth means.
A seventh aspect of this invention is based on the second aspect thereof, and provides a filter device wherein each of the first and second interpolating means comprises N partial interpolating means for implementing double interpolation, and N denotes a predetermined natural number equal to or greater than 2.
An eighth aspect of this invention is based on the third aspect thereof, and provides a filter device wherein each of the first and second logarithmically interpolating means comprises N partial logarithmically interpolating means for implementing double logarithmic interpolation, and N denotes a predetermined natural number equal to or greater than 2.
A ninth aspect of this invention is based on the fifth aspect thereof, and provides a filter device wherein each of the first and second interpolating means comprises N partial interpolating means for implementing double interpolation, and N denotes a predetermined natural number equal to or greater than 2.
A tenth aspect of this invention is based on the sixth aspect thereof, and provides a filter device wherein each of the first and second logarithmically interpolating means comprises N partial logarithmically interpolating means for implementing double logarithmic interpolation, and N denotes a predetermined natural number equal to or greater than 2.
An eleventh aspect of this invention provides a method of acquiring filter coefficients. The method comprises the steps of 1) receiving data of a desired cutoff frequency; 2) deciding whether the desired cutoff frequency is in a predetermined low frequency band or a predetermined high frequency band, the predetermined low frequency band being lower in frequency than the predetermined high frequency band; 3) in cases where the step 2) decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from a first memory section, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in a filtering section; and 4) in cases where the step 2) decides that the desired cutoff frequency is in the predetermined high frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from a second memory section, calculating final filter coefficients from the read-out precalculated basic coefficients through a coefficient expanding process, and setting data of the calculated final filter coefficients in the filtering section.
A twelfth aspect of this invention provides a method of acquiring filter coefficients. The method comprises the steps of 1) receiving data of a desired cutoff frequency; 2) deciding which of a predetermined low frequency band, a predetermined intermediate frequency band, and a predetermined high frequency band the desired cutoff frequency exists in, the predetermined low frequency band being lower in frequency than the predetermined intermediate frequency band, the predetermined intermediate frequency band being lower in frequency than the predetermined high frequency band; 3) in cases where the step 2) decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from a first memory section, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in a filtering section; 4) in cases where the step 2) decides that the desired cutoff frequency is in the predetermined intermediate frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from a second memory section, calculating final filter coefficients from the read-out precalculated basic coefficients through a coefficient expanding process, and setting data of the calculated final filter coefficients in the filtering section; and 5) in cases where the step 2 decides that the desired cutoff frequency is in the predetermined high frequency band, calculating filter coefficients from the desired cutoff frequency, and setting data of the calculated filter coefficients in the filtering section.
A thirteenth aspect of this invention provides a filter device comprising first means for deciding whether a desired cutoff frequency is in a predetermined low frequency band or a predetermined high frequency band, the predetermined low frequency band being lower in frequency than the predetermined high frequency band; second means for storing data of filter coefficients corresponding to different cutoff frequencies in the predetermined low frequency band; third means for storing data of precalculated basic coefficients corresponding to different cutoff frequencies in the predetermined high frequency band, the precalculated basic coefficients being equal to values resulting from a part of calculation to provide final filter coefficients; a filtering section; fourth means for, in cases where the first means decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from the second means, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in the filtering section; and fifth means for, in cases where the first means decides that the desired cutoff frequency is in the predetermined high frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from the third means, calculating final filter coefficients from the read-out precalculated basic coefficients through a coefficient expanding process, and setting data of the calculated final filter coefficients in the filtering section.
A fourteenth aspect of this invention provides a filter device comprising first means for deciding which of a predetermined low frequency band, a predetermined intermediate frequency band, and a predetermined high frequency band the desired cutoff frequency exists in, the predetermined low frequency band being lower in frequency than the predetermined intermediate frequency band, the predetermined intermediate frequency band being lower in frequency than the predetermined high frequency band; second means for storing data of filter coefficients corresponding to different cutoff frequencies in the predetermined low frequency band; third means for storing data of precalculated basic coefficients corresponding to different cutoff frequencies in the predetermined intermediate frequency band, the precalculated basic coefficients being equal to values resulting from a part of calculation to provide final filter coefficients; a filtering section; fourth means for, in cases where the first means decides that the desired cutoff frequency is in the predetermined low frequency band, reading out data of filter coefficients corresponding to the desired cutoff frequency from the second means, and setting the data of the filter coefficients corresponding to the desired cutoff frequency in the filtering section; fifth means for, in cases where the first means decides that the desired cutoff frequency is in the predetermined intermediate frequency band, reading out data of precalculated basic coefficients corresponding to the desired cutoff frequency from the third means, calculating final filter coefficients from the read-out precalculated basic coefficients, and setting data of the calculated final filter coefficients in the filtering section; and sixth means for, in cases where the first means decides that the desired cutoff frequency is in the predetermined high frequency band, calculating filter coefficients from the desired cutoff frequency, and setting data of the calculated filter coefficients in the filtering section.